Radiometric Dating is Inaccurate and Circular in Reasoning
     We have several tools in the dating of fossils.  Some of these are potassium 40, which decays to argon 40, with a half life of 1250 million years.  rubidium 87, which decays to strontium 87, with  a half life of 4880 million years.  And uranium 238, which decays through a series of elements to lead 206, with a half life of 4470 million years.  In conjunction with radiometric dating, we use lithostratigraphy, or rock sequencing.  Then we apply the fossil index.  This gives us three seperate and independant references to the date. 
     As far as inaccuracy to radiometric dating, external factors, such as pressure, temperature, chemicals, cosmic radition, or nearby supernovas, only marginally alter the results.  Supernovas could theoretically alter the results to read too young, not older.  All other experimentation indicates that any other forces could not affect radiation decay by more than 1%.